Duplicate entities are being discarded
What it looks like in your log
Trying to add entity with duplicated UUID 8667ba71-b85a-4004-af54-457a9734eed7. Existing minecraft:cow
What causes it
Entities with identical UUIDs exist, usually after a crash mid-save or copied region files. The server keeps one and drops the rest.
How to fix it
This mostly heals itself. If the log spams it, clean the area with a region tool and stop copying region files between live worlds.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log